Setting laser current based on modulation level of an optical power sensor measurement

ABSTRACT

Before writing to a heat-assisted magnetic recording medium, a DC signal modulated with an AC signal is applied to a laser of a read/write head. A modulation level of an optical power sensor is measured, the optical power sensor being coupled to detect optical output of the laser in response to the modulated current. A target value of the DC signal that causes the modulation levels to reach a predetermined value between zero and a maximum value is determined and used to set a bias current for subsequent activation of the laser based.

RELATED PATENT DOCUMENTS

This application is a continuation of U.S. Ser. No. 15/333,761, filed onOct. 25, 2016, to which priority is claimed and which is incorporatedherein by reference in its entirety.

SUMMARY

The present disclosure is directed to setting laser current based onmodulation level of an optical power sensor measurement. In oneembodiment, before writing to a heat-assisted magnetic recording mediumvia a laser of a read/write head, a laser signal is applied to thelaser. The laser signal includes a DC signal modulated with an ACsignal. In response to different values of the DC signal, modulationlevels are measured of an optical power sensor that is coupled to detectoptical output of the laser in response to the laser signal. A targetvalue of the DC signal is determined that causes the modulation levelsto reach a predetermined value between zero and a maximum value. Thetarget value defines a threshold current above which the laser begins toemit light. A bias current for subsequent activation of the laser is setbased on the threshold current.

In another embodiment, two or more different modulated DC signals areapplied to a laser used for writing data to a heat-assisted magneticrecording medium. Responsive to the two or more different modulated DCsignals, corresponding two or more modulation levels are measured of anoptical power sensor that is coupled to detect an optical output of thelaser. Based on the two or more modulation levels of the optical powersensor, a target modulated DC signal is determined that causes a targetmodulation level of the optical power sensor. A threshold current of thelaser is determined that corresponds to a DC component of the targetmodulated DC signal. A bias current of the laser is set based on thethreshold current.

These and other features and aspects of various embodiments may beunderstood in view of the following detailed discussion and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The discussion below makes reference to the following figures, whereinthe same reference number may be used to identify the similar/samecomponent in multiple figures.

FIG. 1 is a perspective view of a slider assembly according to anexample embodiment;

FIG. 2 is a graph of a laser and optical power sensor response accordingto an example embodiment;

FIG. 3 is a graph showing modulated bias applied to a laser according toan example embodiment;

FIG. 4 is a graph of a AC modulation response of a bolometer accordingto an example embodiment;

FIG. 5 is a graph showing setting of laser bias current according to anexample embodiment;

FIG. 6 is a block diagram of a laser calibration circuit according to anexample embodiment;

FIG. 7 is a block diagram of an apparatus according to an exampleembodiment;

FIG. 8 is a flowchart of a method according to an example embodiment;

FIGS. 9 and 10 are graphs showing temperature-dependent performance ofan apparatus according to an example embodiment;

FIG. 11 is a flowchart of a method according to another exampleembodiment;

FIG. 12 is a block diagram of a preamplifier according to an exampleembodiment; and

In FIGS. 13-16 are graphs illustrating signals associated with thepreamplifier arrangement shown in FIG. 12.

DETAILED DESCRIPTION

The present disclosure generally relates to data storage devices thatutilize magnetic storage media, e.g., disks. Data storage devicesdescribed herein use a particular type of magnetic data storage knownheat-assisted magnetic recording (HAMR), also referred to asenergy-assisted magnetic recording (EAMR), thermally-assisted magneticrecording (TAMR), thermally-assisted recording (TAR), etc. Thistechnology uses an energy source such as a laser to create a smallhotspot on a magnetic disk during recording. The energy from the laseris coupled into an optical waveguide path and directed to a near-fieldtransducer that shapes and directs the energy to heat the recodingmedium. The heat lowers magnetic coercivity at the hotspot, allowing awrite transducer to change magnetic orientation, after which the hotspotis allowed to rapidly cool. Due to the relatively high coercivity of themedium after cooling, the data is less susceptible to data errors due tothermally-induced, random fluctuation of magnetic orientation known asthe superparamagnetic effect.

Because the magnetic field applied during recording is typically muchlarger than the hotspot, the size of the hotspot defines the size of therecorded bits of data. If there are changes to the hotspot size, thenperformance may suffer. For example, if the hotspot is too large, thenadjacent track data may be overwritten. If the hotspot is too small, thecurrent track being recorded will not have sufficient signal andtherefore poor bit error rate (BER).

A number of components and parameters can affect the hotspot size. Oneof these components is the laser diode itself, the output of which canbe changed by changing the drive current. Generally, when the read/writehead is performing non-write activities, such as adjacent track seek ortraversing servo marks, the laser may be kept in a partially activestate from which it can be quickly activated. This involves applying abias current to the laser that causes partial activation, e.g., thelaser emits light at a low level such that its output is insufficient toheat the recording medium but still can be detected.

While the initial biasing current to partially activate the laser can beset in the factory, the properties of the laser change over time and inresponse to environmental conditions, e.g., ambient temperature.Therefore, a data storage device according to an example embodiment willperform regular measurement of the laser's response and, if needed,adjustment of bias current used to achieve the desired level of partialactivation. A bolometer located in or near a light transmission path isused to perform this measurement and adjustment.

In reference now to FIG. 1, a perspective view shows a read/write head100 according to an example embodiment. The read/write head 100 may beused in a magnetic data storage device, e.g., HAMR hard disk drive. Theread/write head 102 may also be referred to herein interchangeably as aslider, write head, read head, recording head, etc. The read/write head100 has a slider body 102 with read/write transducers at a trailing edge104 that are held proximate to a surface of a magnetic recording medium(not shown), e.g., a magnetic disk.

The illustrated read/write head 100 is configured as a HAMR device, andso includes additional components that form a hot spot on the recordingmedium near the read/write transducer 108. These components include anenergy source 106 (e.g., laser diode) and a waveguide 110. The waveguide110 delivers electromagnetic energy from the energy source 106 to anear-field transducer that is part of the read/write transducers 108.The near-field transducer achieves surface plasmon resonance in responseto the coupled light and directs the energy out of a media-facingsurface 112 to create a small hot spot on the recording medium.

An optical power sensor, e.g., bolometer 114, is located at or near alight path (e.g., waveguide 110) to detect emissions from the laserdiode 106. The bolometer 114 may be configured as a wire or otherabsorptive element that has a known change in resistance due to changesin temperature, which is expressed by the temperature coefficient ofresistance (TCR). The bolometer 114 may be selected from materials thatare sensitive to the wavelengths emitted from the laser diode 106, andis coupled to sensing circuitry as known in the art. Generally, theoutput of bolometer 114 will extend from a lower output where the laserdiode 106 is switched off to a higher output where the laser diode 106is activated at an operational power level sufficient to heat therecording medium. It should be noted that the bolometer 114 need not bea special-purpose or separate device. For example, some read/write headsmay use a sensor for measuring temperature changes indicative ofhead-to-media clearance, and this sensor can be used for optical powermeasurements as described herein. In other embodiments, a reader stackand/or or writer coils may be used to detect optical power in responseto temperature. Generally, any device in the read write head whoseresistance changes in response to optical power can be used, assumingsuch resistance change can be measured.

Generally, the laser diode 106 is activated using a DC current. Testingwas performed with a number of bolometer configurations, and it wasfound that using a DC laser bias current resulted in a noisy bolometersignal. However, if the DC signal is modulated, e.g., combined with asine wave, then the signal from the bolometer is less noisy. In FIG. 2,a graph shows measured output of a bolometer according to an exampleembodiment. The photodiode trace 200 represents photodiode measurementsof the laser output as a function of current. Traces 202 and 204represent measured output of a bolometer that is coupled to detect lightfrom the laser over the same current range, where the current wasmodulated with an AC sine wave. The laser current used to obtain trace202 was modulated with a smaller peak-to-peak AC waveform than used toobtain curve 204. This shows that higher levels of AC modulation tend tosmooth out the bolometer response, but both exhibit a similar shape.

In FIG. 3, a graph shows a modulated laser bias signal and associatedbolometer response according to an example embodiment. Curve 300represents the optical output of a laser in response to an applied lasercurrent. As indicated by waveforms 302-304, the laser bias current ismodulated at various current levels. For example, the time varyingequation for the applied modulated current may be I_(mod)=I_(bias)+Bcos(ωt), where I_(bias) is the DC component, B cost(ωt) is themodulating value, and ω is the modulating frequency. In this example, Bis a constant, however B can change at different values of I_(bias). Forexample, B can be set to be some percentage of the initial value ofI_(bias) used at the start of the procedure. The frequency ω may also beconstant, although there may be an optimal range from which ω can beselected. If the frequency is too high, the bolometer may have a poor ACresponse. If the frequency is too low, the level of DC noise will beexcessive.

At Position 1, the bolometer may have a significant DC output, howeverthere is minimal AC output at the target frequency ω since the laser isbelow the threshold current, although noise may induce some small amountof AC output at this frequency. At Position 2, the laser is justbeginning to energize at the laser threshold current, e.g., due topositive peaks of bias current waveform 303 causing light to be emitted.As seen by bolometer waveform 306, this causes peaks at the modulationfrequency ω to be superimposed on the DC signal floor of the bolometer,the DC signal floor being indicated by line 310. The peak value of thewaveform 306 is A/2, where A is a maximum value as seen in FIG. 2.Position 2 is the knee of the laser output curve 300, which occurs atlaser threshold current I_(th).

At Position 2, the A/2 amplitude of the bolometer AC componentcorresponds to half the total amplitude resulting from the modulation ofthe laser current. At currents higher than I_(th) at Position 2, theoptical output along curve 300 is linearly proportional to the laserdiode current, as indicated by slope 312. The slope 312 is also referredto as the slope efficiency, as it represents the change in opticaloutput power ΔP in response to a change in current ΔI. As will bedescribed in greater detail below, the AC modulated laser current andbolometer response can also be used to determine the slope efficiency,which may also change over time, e.g., due to changes in temperature.

At Position 3, the laser bias current is sufficient to cause thebolometer output, as indicated by waveform 307, to be afully-sine-modulated DC signal (e.g., no clipping) with peak-to-peaksine amplitude of A. This is because the lowest value of the waveform304 is at or above the threshold voltage, therefore the laser isilluminated for all values of the current waveform 304. Note that theline 314 thru waveform 307 represents the time-average DC value of thebolometer output. As the modulated laser bias current increases beyondPosition 3, the DC value of the bolometer output will increase, however,the AC component of the bolometer signal will remain about the samebecause the AC modulation component B cost(ωt) of the laser current isconstant. This gives the bolometer AC amplitude response the s-shape asshown in FIG. 2 and discussed further below in relation to FIG. 4.

An example of the bolometer AC modulation amplitude curve 400 is shownin FIG. 4. This curve 400 represents the amplitude of the AC componentof the bolometer signal. The curve 400 is s-shaped, asymptoticallyapproaching zero as the bias current drops below threshold andasymptotically approaching an output level A. At output level A, thebias current becomes large enough so that the modulated laser biascurrent is always above the threshold current and therefore continuouslyactivates the laser. The curve 400 can be used to determine a targetamplitude value at Position 2, the target value being used to adjustlaser bias current I_(bias) just before writing. The target value is ator near the threshold current I_(th) of the laser, and the laser biasI_(bias) may be adjusted to correspond to I_(th) or be different fromI_(th) by a known offset.

In some operational scenarios, it is expected that the value of A oncurve 400 will remain fairly constant during relatively long periods ofoperation. In such a case, the device need not fully plot out the curve400 every time that the laser bias current is set; the last measuredvalue of A can be repeatedly used for multiple settings of bias current.However, the curve 400 may be replotted as needed, e.g., in response tolarge changes in local conditions (e.g., ambient temperature) or inresponse to a timer (e.g., after a predetermined number of hours ofwriting). Between those times, the value of A can be stored in memoryand used to set the bias current as needed, which may occur regularlyduring operation of the device.

For example, during an adjacent track seek (ATS), the laser bias currentcould be checked by applying a modulated laser bias current to the laserand looking at the magnitude of the AC component of the bolometer signalat the target frequency. If the AC component is not at the target level(e.g., A/2), the laser bias is adjusted until the AC magnitude of thebolometer signal is A/2, which is at or near point 402 on curve 400.Point 402 corresponds to Position 2 shown in FIG. 3, which correspondsto the threshold current Ith. In some cases, it may be desired to setthe laser bias to a value that is greater than or less than Ith. Assuch, other values on the curve 400 between zero and A could be used asthe laser bias set point, e.g., A/4, A/3, 2A/3, 3A/4, etc.

In some cases, the system may not have enough time during an adjacenttrack seek to read the AC component of the bolometer, make a correctionand read the AC component again more than once or twice. If the ACcomponent does not converge to a target value during one adjacent trackseek, the process can continue over subsequent adjacent track seeks. Forexample, if ten iterations are needed to reach the correct value and twomeasurement/adjustments can be made per adjacent track seek, it wouldtake five adjacent track seeks to make the correction.

The modulation of the laser bias according to I_(bias)=I_(dc) B cos(ωt)as shown in FIGS. 3 and 4 need only occur during a non-writing eventwhen curve 400 is determined and/or when the laser bias current isadjusted to accommodate for temperature and other changes. At othertimes, a DC current will be applied to the laser (e.g., during writing,at idle). Checking (and, if needed, setting) of the DC bias currentI_(bias) may occur repeatedly during events that occur just beforewriting (e.g., during ATS, long seek, during traversal of a servo mark).When writing commences after the pre-writing event, a predetermined DCoperational offset current I_(OP) is added to DC bias current I_(bias)to fully energize the laser. The value of I_(OP) may be relatively fixedfor a particular time and region of the recording medium, and so settingthe bias current I_(bias) to the correct operating point ensures thecorrect output of the laser during writing in response to currentI=I_(bias)+I_(OP). As will be described in greater detail below, theoptimal value of I_(OP) may change depending on the slope efficiency 312shown in FIG. 3, and the curve 400 shown in FIG. 4 may also be used todetermine slope efficiency and adjust I_(OP) accordingly.

In FIG. 5, a graph shows how the bias current can be checked and setaccording to an example embodiment. In FIG. 5, the curve 400 from FIG. 4is used to represent the response of the bolometer AC modulation as afunction of laser bias current. This curve 400 may have been previouslydetermined before the procedure occurs, although in some cases may bederived as part of the procedure. Line 505 represents a target value onthe curve 400, e.g., A/2. An initial value of the modulated biascurrent, I_(bias)=I_(dc,1)+B cos(ωt) is applied to the laser diode andthe corresponding point 500 is measured. The DC component, I_(dc,1), ofthe modulated current may have been the value of I_(bias) previouslyused for the last writing operation.

Point 500 is too high above the target value 505, and so a bias current,I_(bias)=I_(dc,2)+B cos(ωt) is applied resulting in measured point 501.The value of I_(dc) is further adjusted to obtain points 502 and 503,the value of point 503 being at the desired value along curve 400. TheDC bias value I_(dc,4) corresponding to point 503 is used as the newvalue of I_(bias). Before writing, the modulation is removed and at thestart of writing an operational current I_(OP) is added to the adjustedvalue of I_(bias). For the next non-writing event, e.g., during the nextATS, the current value I_(dc,4) is used as the new starting value andthe illustrated procedure repeats.

The time to perform the adjustment shown in FIG. 5 may be limited bysystem specifications. For example, in one arrangement, about 200 μs isavailable during an ATS to measure the bolometer amplitude from amodulated signal and make at least one adjustment. The measurementshould take no more than half of this time, as the laser may need theremainder of the time to stabilize at full power. In order to quicklymake such a measurement, the procedure shown in FIG. 5 may perform alimited number of iterations. In some configurations, drive preamplifiercircuitry used may utilize specialized hardware to quickly drive thelaser diode with the modulated current and read the bolometer response.

In FIG. 6, a block diagram illustrates a measurement circuit accordingto an example embodiment. A read/write head 600 includes a laser 602 anda bolometer 604 that is coupled to receive light 606 emitted from thelaser 602. The laser 602 is driven by a current source 608 whenactivated by a controller 610. A modulator 612 can be switched on by thecontroller 610 to modulate the current applied to the laser 602 at apredetermined frequency. The bolometer 604 detects the light 606produced by the modulated signal. An AC amplitude detector 614determines an AC component of the bolometer signal at the predeterminedfrequency, and a measure of this AC amplitude is sent to the controller610. The measurement is compared to a reference value of AC amplitudestored in memory 614 (e.g., A/2). Based on the measurement andcomparison, a new value of I_(bias) may be set and stored in the memory614 for at least one subsequent write operation, e.g., until anothermeasurement of bolometer AC amplitude is performed.

A number of circuits may be used as the AC amplitude detector 614. Forexample, a diode and capacitor placed in series will output a DC voltage(measured across the capacitor) equal to the peak value of an AC signalapplied to the series diode and capacitor. This is known as a peakdetector or envelope detector. In other embodiments, a lock-in amplifiercan be used. A lock in amplifier can extract the amplitude of the ACcomponent at the target frequency from a noisy environment.

In other embodiments, digital signal processing circuits can be used todetect AC amplitude at the target frequency. For example, a simple sinewave has no harmonics, and so a transform of the sine wave to frequencydomain (e.g., using a Fast Fourier Transform, or FFT) will appear as animpulse/delta function at the frequency of the sine wave. However, asindicated by curve 306 in FIG. 3, if the laser is biased at thethreshold current, the output of the bolometer may appear as a clippedsine wave. The distortion amplitudes relative to the main peak in thespectrum may be used to find the target operating point. Hard drives mayalready include a harmonic sensor that can measure two frequencies atonce (e.g., for determining head-to-media clearances). The bolometeroutput may be routed to harmonic sensing circuitry in the preamplifierto determine the main peak and a distortion peak.

In FIG. 7, a diagram illustrates components of a data storage device 700(e.g., hard disk drive apparatus) according to an example embodiment.The apparatus includes circuitry 702 such as a system controller 704that processes read and write commands and associated data from a hostdevice 706. The host device 706 may include any electronic device thatcan be communicatively coupled to store and retrieve data from a datastorage device, e.g., a computer. The system controller 704 is coupledto a read/write channel 708 that reads from and writes to surfaces ofone or more magnetic disks 710.

The read/write channel 708 may include analog and digital circuitry suchas decoders, timing-correction units, error correction units, etc. Theread/write channel is coupled to the heads via interface circuitry 713that may include preamplifiers, filters, digital-to-analog converters,analog-to-digital converters, etc. The read/write channel 708 generallyconverts data between the digital signals processed by the systemcontroller 704 and the analog signals conducted through two or moreread/write heads 712 during read operations. At least one of theread/write heads 712 includes a laser used to heat a spot on themagnetic disk 710 during recording of data. A laser control module 709controls various aspects of the laser operation during both reading andwriting. The laser module 709 may dictate when the laser is switch onand off, set laser current based on various conditions, monitor laserpower, e.g., via a bolometer on the read/write head 712. Generally, theread/write channel 708 provides facilities for communicating this andother control/sensor data with the read/write heads 712.

In addition to processing user data, the read/write channel 708 readsservo data from servo wedges 714 on the magnetic disk 710 via theread/write head. All of the multiple readers of the read/write head maybe used to read servo data, or only a subset thereof. The servo data aresent to a servo controller 716, which uses the data to provide positioncontrol signals 717 to a VCM 718. The VCM 718 rotates an arm 720 uponwhich the read/write heads 712 are mounted in response to the controlsignals 717. The position control signals 717 may also be sent tomicroactuators 724 that individually control each of the read/writeheads 712, e.g., causing small displacements at each head.

A laser calibration module 728 performs a calibration that includesmodulating a bias current applied to a read/write head laser and readingthe signal back via a bolometer and other signal processing circuits asdescribed above. The timing of the calibration may be coordinated withthe servo controller 716. For example, the servo controller 716 commandsthe read/write heads 712 to move to a new track and detects when theread/write heads are positioned over the target track, and so can beused to provide timing signals for the laser calibration. Other eventsdetected by the servo controller 716 for purposes of scheduling a lasercalibration include a long seek, and traversal of servo marks.

In FIG. 8, a flowchart shows a method of setting a laser currentaccording to an example embodiment. The method involves determining 800an event before writing with the laser. The laser is part of aread/write head that is used to write to a heat-assisted magneticrecording medium. The setting of the laser current involves modulating801 a current of the laser about a default threshold current level,e.g., a previously measured laser threshold current. A modulation levelof a bolometer that is coupled to detect optical output of the laser ismeasured 802. The modulation level is the magnitude of the AC componentof the bolometer signal. A bias current is adjusted 803 for subsequentactivation of the laser based on the modulation level of the bolometer.The laser may be activated for writing data 804 using the adjusted biascurrent. For example, a predetermined current may be added to the biascurrent to achieve a desired optical power for recording data. The biascurrent may be the same as or different from the threshold current.

The methods and apparatuses described above were tested in order todetermine effectiveness over a range of temperatures. In FIG. 9, a setof graphs shows results of tests on an apparatus configured according toan embodiment described above. The top graph shows optical output of alaser in response to current over a range of temperatures. The bottomgraph shows measurements of AC component magnitude from a bolometer forthe same range of currents and temperatures. As seen by points 900 and901 in the top graph, the threshold current increases with thetemperature. This increase in threshold current is also seen in thebottom graph at corresponding points 902 and 903. Therefore, thevertical midpoints of the bottom curves accurately predict the thresholdcurrent across a range of temperature.

As indicated by amplitude differences A_(29° C.) and A_(62° C.) in thebottom graph of FIG. 9, the bolometer response changes as a function oftemperature. This can be used to determine if and when a new calculationof the value of A may be needed. For example, the value of A for amid-temperature curve (e.g., at 45° C.) may be used if the resultingerror in the calculation of threshold current is acceptable. In othercases, a target error in calculating threshold current can be used toderive a temperature change that results in recalculating the bolometercurve. For example, if ambient temperature changes by 10° C. from whenthe last curve was derived, a new curve can be run and the value of Adetermined from that curve.

While the above embodiments describe using the vertical difference Abetween the asymptotic parts of the bolometer AC component response, thehorizontal difference between the asymptotic parts can also be used todetermine slope efficiency. In reference again to FIG. 4, location ofPosition 3 is indicative of the slope efficiency of the laser (e.g.,slope efficiency as indicated by slope 312 in FIG. 3) For example,assume that the difference in optical output power ΔP of the laser (seeFIG. 3) that induces the change in amplitude A shown curve 400 isrelatively constant as a function of temperature. Therefore, the slopeefficiency M can be calculated as M=ΔP/ΔI, where ΔI is determined fromthe curve 400 of FIG. 4.

Using the measurements used to obtain the data shown in the graphs ofFIG. 9, slope efficiency was calculated as shown in the graph of FIG.10. As expected, the slope efficiency decreases as temperatureincreases, and does so fairly linearly. This result can be used tochange the laser operational current I_(OP) in response to changes inambient temperature. For example, if I_(OP) is to induce a desiredchange in power level ΔP_(OP), then I_(OP)=ΔP_(OP)/M_(amb), whereM_(amb) is the slope efficiency measured at the current ambienttemperature using the bolometer response described above. It will beunderstood that the slope efficiency can be recalculated for otherconditions besides change in ambient temperature, such as aging of thelaser.

In FIG. 11, a flowchart illustrates a method according to anotherexample embodiment. The method involves applying 1101 at least twodifferent modulated DC currents to a laser used in writing data to aheat-assisted magnetic recording medium. For example a series ofmodulated currents in the form of I_(dc)+B cos(ωt) can be applied fordifferent values of I_(dc), e.g., where B and ω are constant. First andsecond modulation levels of a bolometer are measured 1102. The bolometeris coupled to detect an optical output of the laser in response to theat least two modulated DC currents. The first and second modulationlevels respectively correspond to a first of the modulated DC currentsthat causes the laser to begin emitting light (e.g., Positions 1 or 2 inFIG. 4) and a second of the modulated DC currents that is large enoughto continuously activate the laser (e.g., Position 3 in FIG. 4). A slopeefficiency of the laser is determined 1103 based on the first and secondmodulated DC currents. A laser operational current T_(OP) can optionallybe determined and/or adjusted 1104 based on the slope efficiency.

The methods described above can be implemented in a preamplifiercircuit. Several features described below may be enabled via firmwareand/or setup (e.g., grounding of pins). In one embodiment, the preampwould allow the user to enable laser modulation internally (or sourceexternally), enable automatic laser bias increments controlled viaexternal logic to preamp, enable the AC sensing path of the bolometer,and enable an demodulator/envelop detector with automatic digitizationscheme to enable to the capture of the laser current sweep as seen bythe AC path of the TCR sensor. In FIG. 12, these features are shown in ablock diagram illustrating a preamplifier 1200 according to an exampleembodiment.

As seen in FIG. 12 the modulation can be provided externally to thepreamplifier 1200, as indicated by oscillator 1204 in system-on-a-chip(SoC)/controller 1202. The modulation could also be internal to thepreamplifier 1200 to save pins and external hardware lines, as indicatedby oscillator 1206. Switches 1208 allow selecting between internal andexternal modulation. The modulation is applied to laser diode 1210 vialaser driver. The laser current can be auto incremented at triggerpoints controlled by the rising edge of an external logic line 1212. Asensor path 1214 includes an amplifier 116 and bandpass filter 1218 toprocess incoming signals from a bolometer 1220. The sensor path 1214monitors the response from the bolometer 1220 during laser modulationand laser bias auto increment.

An envelop detector 1222 is coupled to the sensor path 1214. The envelopdetector 1222 acts as a demodulator to get an average energy of theincoming signal from the AC path 1214. The envelop detector includes arectifier 1226 whose output is fed into a low-pass filter 1224. Theoutput of the envelop detector/demodulator would then be used as aninput to the either an onboard analog-to-digital converter (ADC) 1226 orother off-chip ADC circuits 1228. The on-chip ADC 1226 can be digitizedat specific intervals determined by the switching of the same externallogic signal lines 1212 controlled by the SOC 1202. For example, if theincrementing and application of laser current occurs in response to arising edge from the logic signal lines 1212, the acquisition by ADC1226 can be triggered by a falling edge of the same signal lines 1212.

In FIGS. 13-16, graphs illustrate signals associated with thepreamplifier arrangement shown in FIG. 12. The system firmware can setupthe preamplifier 1200 to enable the laser threshold measurement assist.Once this is done the laser I_(bias) auto-increments around servo wedgevia a hardware line toggle edge detection until it reaches a programmedfinal value. The servo gate signal is shown in the graph of FIG. 13, andthe laser bias signal is shown in the graph of FIG. 14. During theauto-increment of the mean I_(bias) the signal applied to the laser 1210is modulated about I_(bias) either using on-chip or off-chip modulation.Once the final value is reached the preamplifier assist is turned offand the laser drive returns to normal operation.

As the sweep from low to high I_(bias) occurs, the sensor path 1214senses modulation from the laser 1210 as a voltage or current across thesensor 1220. An example of the sensor signal is shown in the graph ofFIG. 15. This signal is amplified bandpass-filtered and sent to thedemodulator/envelop-detector 1222. The graph of FIG. 16 shows an exampleof the envelop detector output. The dots on the envelop detector curvein FIG. 16 represent the value of digitized sample sent toSoC/controller 1202. These samples are used for the aforementionedthreshold, slope efficiency and other metrics of laser power.

The foregoing description of the example embodiments has been presentedfor the purposes of illustration and description. It is not intended tobe exhaustive or to limit the embodiments to the precise form disclosed.Many modifications and variations are possible in light of the aboveteaching. Any or all features of the disclosed embodiments can beapplied individually or in any combination are not meant to be limiting,but purely illustrative. It is intended that the scope of the inventionbe limited not with this detailed description, but rather determined bythe claims appended hereto.

What is claimed is:
 1. A method, comprising: before writing to aheat-assisted magnetic recording medium via a laser of a read/writehead, applying a laser signal to the laser, the laser signal comprisinga DC signal modulated with an AC signal; measuring, in response todifferent values of the DC signal, modulation levels of an optical powersensor that is coupled to detect optical output of the laser in responseto the laser signal; determining a target value of the DC signal thatcauses the modulation levels to reach a predetermined value between zeroand a maximum value, the target value defining a threshold current abovewhich the laser begins to emit light; and setting a bias current forsubsequent activation of the laser based on the threshold current. 2.The method of claim 1, wherein the subsequent activation of the lasercomprises adding an operational current to the bias current to fullyactivate the laser for writing to the heat-assisted magnetic recordingmedium.
 3. The method of claim 2, further comprising: determining aslope efficiency based on the modulation level of the optical powersensor; and modifying the operational current based on the slopeefficiency.
 4. The method of claim 1, wherein the modulation level ofthe optical power sensor as a function of the different values of the DCsignal comprises an s-shaped curve asymptotically approaching zero asthe laser signal becomes small enough not to activate the laser andapproaching the maximum value as the laser signal becomes large enoughto continuously activate the laser.
 5. The method of claim 4, whereinthe predetermined value of the modulation levels is between ¼ and ¾ ofthe maximum value.
 6. The method of claim 1, wherein the optical powersensor detects the optical output of the laser based on detecting atemperature proximate a light delivery path of the read/write head. 7.The method of claim 1, wherein the method is performed during anadjacent track seek that occurs before writing to the heat-assistedmagnetic recording medium.
 8. A method comprising: applying two or moredifferent modulated DC signals to a laser used for writing data to aheat-assisted magnetic recording medium; responsive to the two or moredifferent modulated DC signals, measuring corresponding two or moremodulation levels of an optical power sensor that is coupled to detectan optical output of the laser; based on the two or more modulationlevels of the optical power sensor, determining a target modulated DCsignal that causes a target modulation level of the optical powersensor; determining a threshold current of the laser that corresponds toa DC component of the target modulated DC signal; and setting a biascurrent of the laser based on the threshold current.
 9. The method ofclaim 8, wherein the setting of the bias current occurs during theoperation of a magnetic storage device and compensates for changes inproperties of the laser over time.
 10. The method of claim 8, whereinapplying the two or more different modulated DC signals comprisesapplying two or more currents in the form of I_(dc)+B cos(ωt) fordifferent values of I_(dc), where B and ω are constant.
 11. The methodof claim 8, further comprising, after setting the bias current, addingan operational current to the bias current to fully activate the laserfor writing to the heat-assisted magnetic recording medium.
 12. Themethod of claim 11, further comprising: determining a slope efficiencyof the laser based on the two or more modulation levels of the opticalpower sensor; and modifying the operational current based on the slopeefficiency.
 13. The method of claim 8, wherein the optical power sensorcomprises a bolometer.
 14. An apparatus comprising: circuitry thatinterfaces with: a laser of a read/write head, the laser using inwriting data to a heat-assisted magnetic recording medium; and anoptical power sensor that is coupled to detect optical output of thelaser; and a controller coupled to the circuitry and configured toperform a procedure comprising: before writing to the heat-assistedmagnetic recording medium, applying a laser signal to the laser, thelaser signal comprising a DC signal modulated with an AC signal;measuring, in response to different values of the DC signal, modulationlevels of the optical power sensor; determining a target value of the DCsignal that causes the modulation levels to reach a predetermined valuebetween zero and a maximum value, the target value defining a thresholdcurrent above which the laser begins to emit light; and setting a biascurrent for subsequent activation of the laser based on the thresholdcurrent.
 15. The apparatus of claim 14, wherein the subsequentactivation of the laser comprises adding an operational current to thebias current to fully activate the laser for writing to theheat-assisted magnetic recording medium.
 16. The apparatus of claim 15,wherein the procedure further comprises: determining a slope efficiencybased on the modulation level of the optical power sensor; and modifyingthe operational current based on the slope efficiency.
 17. The apparatusof claim 14, wherein the modulation level of the optical power sensor asa function of the different values of the DC signal comprises ans-shaped curve asymptotically approaching zero as the laser signalbecomes small enough not to activate the laser and approaching themaximum value as the laser signal becomes large enough to continuouslyactivate the laser.
 18. The apparatus of claim 17, wherein thepredetermined value of the modulation levels is between ¼ and ¾ of themaximum value.
 19. The apparatus of claim 14, wherein the optical powersensor detects the optical output of the laser based on detecting atemperature proximate a light delivery path of the read/write head. 20.The apparatus of claim 14, wherein the procedure is performed during anadjacent track seek that occurs before writing to the heat-assistedmagnetic recording medium.